The name 'Sunflower' itself indicates that it always faces the Sun. And it’s widely speculated that sunflowers always face the sun no matter wherever the sun lies.
A sunflower contains a growth hormone called auxin that is sensitive to sunlight. Therefore, when sun shines, the auxin moves towards the shaded portion of the plant. Thus, making the flower head face the sun.
To maximize photosynthesis, sunflowers like many flowers follow a repetitive natural solar tracking behaviour, called heliotropism.
A video made by biologist Roger Hangarter over a 24 hour period showed that a sunflower is facing east while the sun was setting in the west. Hangarter states that this is due to the flower anticipating the next day’s sunrise. When the sun rises next day, the flower head follows the sunlight as auxins move to the opposite direction.
On sunny days, young flower buds of the sun will follow the sun from east to west and return to face east at the end of day. But, as flower buds grows and matures, the stem becomes strong and flower gets permanently stuck facing eastward.
Therefore, a sunflower do follow the sun when it is young because of a growth hormone called auxin. The hormone sensitive to sunlight moves to a shaded area of the flower when sunshines. Sunflowers follows a natural repetitive solar tracking movement. However, sunflowers can be seen facing east during sunset as it waits for the next day’s sunrise. Young sunflowers do follow the sun but matured flowers gets stuck facing the east as the stem becomes strong. Hence, it is mostly-true that sunflowers always face the sun.